We have completed and published a study on the role of fibronectin in the migration of primordial germ cells of Xenopus. We are now analyzing its role in earlier stages of amphibian embryogenesis. We have carried out cross-linking studies with the radioactive cross-linking reagent developed last year. We have identified a new interaction of fibronectin (with thrombospondin) during platelet adhesion to surfaces. We have carried out further analyses of this interaction by other methods. We are now in the process of a detailed analysis of platelet-surface and -secreted proteins and their interaction. We also are using the cross-linking reagent in other studies on fibroblast adhesion and on the receptors for several macromolecules which bind to surface membranes. Various aspects of endocytosis by macrophages and fibroblasts are now being analyzed biochemically. We will continue with analyses of the biological systems we have been studying, using biochemical methods including cross-linking, immunological probes and cell biological methods. Our primary aim will be to study the nature and role of protein-protein interaction in these systems.